In recent years, energy harvesting devices (environmental power generation elements) such as wireless sensors or remote control switches that operate without wiring or battery replacement by obtaining electrical energy from a surrounding environment due to realization of low power consumption of an electronic circuit or a wireless technology have attracted attention. Therefore, for example, a low-illuminance dye-sensitized solar battery for energy harvesting assumed to be used in indoor light of a fluorescent light, LED illumination, or the like has been developed.
There is a power supply device using a related lithium ion capacitor (see Patent Document 1). The power supply device described in Patent Document 1 is a power supply device including a lithium ion capacitor, and includes a power control unit that operates the lithium ion capacitor in a voltage range from 2.0 V to 3.2 V.
Further, Patent Document 1 describes that an appropriate amount of power generation required for a solar battery is obtained on the basis of a power demand amount, and the amount of power generation of a solar battery is adjusted according to a required generated power amount.
Further, attempts have been made to cause a solar battery to generate power under a low-illuminance environment such as indoor light, accumulate the generated power in a storage battery, and drive a load device with the stored power. In this case, it is desirable to use a lithium ion capacitor as the storage battery since the lithium ion capacitor has a large capacitance and a low leakage current.
Lithium ion capacitors that are commercially available are mainly lithium ion capacitors with 40 F (farads) or more, such as 40 F or 100 F. Further, it is preferable for a lithium ion capacitor to be used at a voltage of 2.0 V or higher from the viewpoint of prevention of deterioration of cells, as described in Patent Document 1. Therefore, a power supply device prevents a voltage of a lithium ion capacitor from becoming a voltage of 2.5 V or lower, for example, in consideration of a margin. Therefore, when a charging voltage of a lithium ion capacitor decreases and is lower than 2.5 V, an operation of a load device is temporarily stopped and supply of power is stopped. Thereafter, in the power supply device, when the power generation element starts power generation, recharging of the lithium ion capacitor is started by the power generation element.